1. Field of the Invention
This invention relates to circuit breakers having a thermal-magnetic trip mechanism, and in particular to such a circuit breaker which utilizes the bimetal of the thermal-magnetic trip mechanism for sensing current, such as for example, for an arcing fault detector, a meter or an overcurrent alarm.
2. Background Information
It has been observed that arcing type faults in a power distribution system typically do not trip the conventional circuit breaker. An arcing fault can be caused by bared conductors which intermittently touch and then separate due to magnetic forces generated by the fault current. Thus, the arcing fault is characterized by repetitive step increases in current. Such faults have a high resistance, and being intermittent they draw limited current and therefore do not trip the conventional circuit breaker. However, they can start a fire if combustible material is present.
If the settings of the conventional trip unit are lowered so that they trip in response to an arcing fault, an unacceptable number of nuisance trips can be induced by a number of common loads which generate a similar current step function.
A great deal of effort has been directed toward developing a circuit breaker which can respond to arcing faults in a power distribution system but be immune to nuisance tripping. One approach has been to analyze the current waveform produced by the arcs to identify unique arc characteristics. However, such detectors tend to be expensive, especially for the miniature circuit breakers typically used in residential and light commercial applications.
Commonly owned U.S. Pat. No. 5,224,006 discloses a circuit breaker which utilizes a bandwidth limited di/dt signal to detect arcing faults. The di/dt signal is generated by a sensing coil shared with a dormant oscillator type ground fault detector. Pending U.S. application Ser. No. 08/023,435 filed on Feb. 26, 1993, improves discrimination of arcing faults from current transients caused by commonly encountered loads by recognizing that such transients caused by the loads are typically singular events while the arcing fault is repetitive. Hence, this patent application discloses an arcing fault circuit breaker which only trips upon detection of two step increases in current within about a second. This patent application also suggests that as an alternative to the use of a detector coil to generate the di/dt signal, the resistance across a section of wire carrying the load current can be used to measure the current. In order to generate a signal of sufficient magnitude in a reasonable length of wire, the gauge of the wire had to be reduced. However, the smaller gauge wire was not able to sustain typical short circuit currents.
A typical thermal-magnetic circuit breaker has a bimetal which heats up and bends to unlatch a spring operated trip mechanism in response to sustained overcurrents. The magnetic trip is provided by an armature which is a magnetically attracted by current flowing through the bimetal to unlatch the trip mechanism in response to short circuit currents.
At present there is no simple way of determining the current flowing through a circuit breaker with a thermal-magnetic trip. It would be useful to be able to easily measure this current, either just to provide an accurate determination of the load current, or for use in devices responsive to current, such as for example, an arcing fault detector or an overload alarm.
There is a need for an improved circuit breaker with a thermal-magnetic trip device having a simple, inexpensive arrangement for determining current passing through the circuit breaker.
There is also a need for an improved circuit breaker providing protection against arcing faults which is reliable and inexpensive.
There is a further need for such an improved circuit breaker which does not require a coil to generate a di/dt signal for use in detecting arcing faults, yet can withstand typical short circuit currents.
There is a further need for such a circuit breaker in which the arcing fault detector requires a minimum of space.